FIG. 1 shows an example of a communications system 100. The communications system 100 comprises a UE 101 which is served by a Radio Access Network (RAN) node. The RAN node is represented by Evolved Universal Terrestrial Access Network (E-UTRAN) 103 in FIG. 1. The RAN node may be for example a base station, a NodeB, an evolved NodeB (eNode B, eNB), Radio Network Controller (RNC) or any other element capable to communicate with a UE 101. The reference point between the UE 101 and the E-UTRAN 103 may be referred to as Long Term Evolution-Uu (LTE-Uu).
A Mobility Management Entity (MME) 105 may be connected to the E-UTRAN 103 via the reference point S1-MME. The MME 105 is an element having functions such as e.g. Non-Access Stratum (NAS) signalling, Inter Core Network (CN) node signalling for mobility between Third Generation Partnerhip Project (3GPP) access networks, UE reachability, Tracking Area (TA) list management, Packet data network Gateway (PGW, PDN GW) and Serving GateWay (SGW) selection, MME selection for handover with MME change etc. S10 is the reference point between a plurality of MMEs 105 for MME relocation and MME to MME information transfer. The MME 105 manages and stores UE context information (for idle state: UE/user identities, UE mobility state, user security parameters etc.).
The Home Subscriber Server (HSS) 108 is a subscriber server node similar to the Global System for Mobile Communications (GSM) Home Location Register (HLR) and Authentication Centre (AuC). The HSS 108 comprises subscriber-related information (subscriber profiles), performs authentication and authorization of the user, and may provide information about the subscriber's location and IP information. The reference point S6a enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system between the MME 105 and the HSS 108.
Two gateways are seen in FIG. 1, i.e. the SGW 110 and the PGW 118. The SGW 110 and the PGW 118 may be implemented in one physical node or in separate physical nodes. The SGW 110 is the gateway which terminates the interface towards E-UTRAN 103. The reference point between the SGW 110 and the E-UTRAN 103 for the per bearer user plane tunneling and inter eNodeB path switching during handover may be referred to as S1-U. The SGW 110 routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (relaying the traffic between 2G/3G systems and the PGW 118) etc. S11 is the reference point between the SGW 110 and the MME 108.
The PGW 118 is the gateway which terminates the SGi interface towards the Packet Data Network (PDN). The PDN is illustrated in FIG. 1 by the Operator's IP Services (e.g. IMS, PSS etc.) 119. IP is short for Internet Protocol, IMS is short for IP Multimedia Subsystem or IM Multimedia core network Subsystem and PSS is short for Packet Switched Streaming. If the UE 101 is accessing multiple PDNs, there may be more than one PGW 118 for that UE 101. Functions of the PGW 118 are e.g. providing connectivity from the UE 101 to external PDNs by being the point of exit and entry of traffic for the UE 101, performing policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening etc. S5 is the reference point which provides user plane tunnelling and tunnel management between the SGW 110 and the PGW 118.
The SGSN 120 is responsible for the delivery of data packets from and to the UE's 101 within its geographical service area. SGSN 120 is short for Serving GPRS Support Node, where GPRS is short for General Packet Radio Services. One of the SGSN's 120 functions is to provide signaling for mobility between 2G/3G and E-UTRAN 103 3GPP access networks. 2G/3G access network are exemplified with GERAN 122 and UTRAN 125 in FIG. 1. GERAN is short for GSM EDGE Radio Access Network where EDGE is short for Enhanced Data Rates for GSM Evolution. UTRAN is short for Universal Terrestrial Radio Access Network.
Some further functions of the SGSN 120 are to handle packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions etc. S3 is the interface between the SGSN 120 and the MME 105. S4 is a reference point between the SGSN 120 and the SGW 110. S12 is the reference point between the SGW 110 and the UTRAN 125. In some embodiments, the SGSN 120 and the MME 105 are co-located in one node. In this text, the term MME/SGSN will refer to any one of a standalone MME 105 or a standalone SGSN 120 or a combined MME 105 and SGSN 120 node.
The Policy and Charging Rules Function (PCRF) 130 is a policy and charging control element. The PCRF 130 encompasses policy control decision and flow based charging control functionalities, it provides network control regarding the service data flow detection, gating, Quality of Service (QoS) and flow based charging etc. The PCRF 130 may be described as a functional entity which may be a standalone node or a function implemented in another node. The reference point Gx provides transfer of (QoS) policy and charging rules from the PCRF 230 to a Policy and Charging Enforcement Function (PCEF) in the PGW 118.
Rx is the reference point which resides between the PCRF 130 and the Operator's IP Services 119. The Rx reference point is used to exchange application level session information between the PCRF 130 and the Application Function (AF) (not shown).
In some embodiments, a communications system may be divided into a Radio Access Network (RAN) and a Core Network (CN). The RAN may be e.g. the E-UTRAN 103 and may comprise a RAN node such as e.g. the base station as described above. Using the exemplary embodiment in FIG. 1, the CN may comprise for example the MME 105, the SGW 110, the PGW 118, the SGSN 120, the HSS 108 and the PCRF 130. The RAN and the CN may each comprises additional entities not shown in FIG. 1. The CN may be a Packet Switched (PS) core network or a Circuit Switched (CS) core network.
It should be noted that the communication links in the communications systems seen in FIG. 1 may be of any suitable kind including either a wired or wireless link. The link may use any suitable protocol depending on type and level of layer (e.g. as indicated by the Open Systems Interconnection (OSI) model) as understood by the person skilled in the art.
Network Functions Virtualization (NFV) (also known as Virtual Network Function (VNF)) is defined by the European Telecommunications Standards Institute (ETSI) as the “principle of separating network functions from the hardware they run on by using virtual hardware abstraction”. Together with Software Defined Network (SDN), NFV is a hot topic in the telecommunication technology field, and aims at providing new ways of designing, building and operating networks.
NFV is also applicable to the Evolved Packet Core (EPC). vEPC is the abbreviation used when referring to NFV applied to the EPC. vEPC is short for Virtualized Evolved Packet Core or Virtual Evolved Packet Core. vEPC involves virtualization of at least some of the native EPC components. Such virtualized components may be for example a virtualized Mobility Management Entity (vMME), virtualized SGSN (vSGSN), virtualized PGW (vPGW), virtualized Gateway GPRS Support Node (vGGSN), virtualized PCRF (vPCRF) etc. The vEPC is fully compatible with the native EPC, and many native and virtual EPC nodes may coexist in a network.
In a stateless architecture for the core network (e.g. the EPC or vEPC) the control logic will be stateless and the UE states (the UE states may be referred to as UE Context) will be saved persistently in an external database. The control logic serving the UEs will be interchangeable and it reads the UE states only at invoking and execution of well-defined procedures. Stateless may be described as there is no memory of the past and that every transaction, interaction or event is performed as if it were being done for the very first time. Stateful is the opposite of stateless and may be described as there is memory of the past and that previous transactions, interactions or events are remembered and may affect the current transaction. A stateless system does not have any records of previous interactions and each interaction request has to be handled based on information that comes with the request. A stateful system, on the contrary, keeps track of the state of interaction, for example by setting values in a storage field dedicated for this purpose.
Mobility management is a function of communications networks which are necessary for being able to track where UEs are located, allow calls, data transfer and other services to be delivered to and from the UEs. Procedures such as Tracking Area Update (TAU) and Routing Area Update (RAU) are part of the mobility management of the EPC/LTE system and GERAN/UTRAN system, respectively.
A TAU or RAU is initiated by the UE when it detects a new TA outside its current TA list, or when it detects a new Routing Area (RA). TAU involves entities such as e.g. the MME and RAU involves entities such as e.g. the SGSN.
An example of TAU procedure will now be described. FIG. 2 is a signaling diagram illustrating an example of a TAU procedure with a SGW change, based on the communications system exemplified in FIG. 1. For TAU, the UE 101 will be in IDLE mode and when passing a cell border (the cell border is also a border between TAs) and the new TA is not part of the TA list, the UE will send a TAU message to the MME. The TAU message comprises the ID of the new TA associated with the UE. The MME 105 will receive the TAU message and saves the new TA ID associated with the UE. The MEM 105 will also update other parts of the system that needs location updating e.g. the HSS 128 or the SGW 110. The RAN 103 is represented by an eNB in FIG. 2. The reference numbers with the letter a refers to an old node, i.e. a node in the old TA. The reference numbers with the letter b refers to a new node, i.e. a node in the new TA. Optional steps are indicated with dotted arrows.
The method in FIG. 2 comprises at least some of the following steps, which steps may be performed in any suitable order than described below:
Step 201
A trigger for starting the TAU procedure occurs.
Step 202
The UE 101 initiates the TAU procedure by sending, to the eNB 103, a TAU Request message.
Step 203
The eNB 101 derives or selects the new MME 105b from the parameters comprised in the TAU Request message in step 202. The eNB 103 forwards the TAU Request message together to the new MME 105b. The new MME 105b is associated with the new TA to which the UE 101 has moved.
Step 204
The new MME 105b differentiates the type of the old node, i.e. MME 105 or SGSN 120, and uses information received from the UE 101 to derive the old MME 105a or the old S4 SGSN 120a address, and sends a Context Request message to the old MME/old S4 SGSN 105a, 120a to retrieve user information.
Step 205
If the Context Request message is sent to an old MME 105a, the old MME 105a responds with a Context Response message to the new MME 105a. If the Context Request is sent to an old S4 SGSN 120a, the old S4 SGSN 120a responds with a Context Response to the new MME 105a. 
Step 206
If the integrity check of TAU Request message (sent in step 202) failed, then authentication is mandatory. If the integrity check of the TAU Request message succeeded, then the authentication is optional. The authentication involves the UE 101, the new MME 105b and the HSS 108,
Step 207
The SGW 110 is relocated when the old SGW 110a cannot continue to serve the UE 101. The MME 105 (if the MME has changed then it is the new MME 105b) may also decide to relocate the SGW 110 if a new SGW 105b is expected to serve the UE 101 longer and/or with a more optimal UE 101 to PGW path, or if a new SGW 105b can be co-located with the PGW 115. If the MME 105 has changed the new MME 105b sends a Context Acknowledge message with a SGW change indication to the old MME/old S4 SGSN 105a, 120a. 
Step 208
If the MME 105 has changed, the new MME 105b verifies the Evolved Packte System (EPS) bearer status received from the UE 101 with the bearer contexts received from the old MME/old S4 SGSN 105a, 120a. If the MME 105 has not changed, the MME 105 verifies EPS bearer status from the UE 101 with the bearer contexts available in the MM context. The MME 105 releases any network resources related to EPS bearers that are not active in the UE 101. If there is no bearer context at all, the MME 105 rejects the TAU Request. If the MME 105 selected a new SGW 110b, it sends a Create Session Request message per PDN connection to the selected new SGW 110b. 
Step 209
The new SGW 110b informs the PGW(s) 115 about the change of for example the Radio Access Technology (RAT) type that e.g. can be used for charging, by sending the message Modify Bearer Request per PDN connection to the PGW(s) 115 concerned.
Step 210
If dynamic PCC is deployed, and RAT type information needs to be conveyed from the PGW 115 to the PCRF 130, then the PGW 115 shall send RAT type information to the PCRF 130 by means of an Internet Protocol Conectivity Access Network (IP CAN) Session Modification procedure.
Step 211
The PGW 115 updates its bearer contexts and returns a Modify Bearer Response message to the new SGW 110b. 
Step 212
The new SGW 110b updates its bearer context. This allows the new SGW 110b to route bearer PDUs to the PGW 115 when received from the eNodeB 103. The new SGW 110b returns a Create Session Response message to the new MME 105b. 
Step 213
If there are no subscription data in the new MME 105b for this UE 101, or for some network sharing scenario if the Public Land Mobile Network-Identity (PLMN-ID) of the Tracking Area Identity (TAI, TA ID) supplied by the eNodeB is different from that of the Globally Unique Temporary Identity (GUTI) in the UE's context, then the new MME 105b sends an Update Location Request message to the HSS 108.
Step 214
The HSS sends the message Cancel Location message to the old MME 105b with Cancellation Type set to Update Procedure.
Step 215
If a timer started in step 204 is not running, the old MME 105a removes the MM context. Otherwise, the contexts are removed when the timer expires. It also ensures that the MM context is kept in the old MME 105a for the case the UE 101 initiates another TAU procedure before completing the ongoing TAU procedure to the new MME. The old MME 105b acknowledges with the message Cancel Location Acknowledgement (Ack).
Step 216
When old S4 SGSN 120b receives the Context Acknowledge message and if the UE 101 is in lu Connected, the old S4 SGSN 120b sends an lu Release Command message to a RNC 135 after the timer started in step 204 has expired. This is an optional step, as indicated with the dotted arrow.
Step 217
The RNC 135 responds with an lu Release Complete message sent to the old S4 SGSN 120b. This is an optional step, as indicated with the dotted arrow.
Step 218
The HSS 108 acknowledges the Update Location Request message from step 213 by sending an Update Location Ack message to the new MME 105b. 
Step 219
If the MME 105 has changed, when a timer started in step 204 expires, the old MME/old S4 SGSN 105a, 120a releases any local MME 105 or SGSN 120 bearer resources and additionally the old MME/old S4 SGSN 105a, 120a deletes the EPS bearer resources by sending the Delete Session Request messages to the old SGW 110a if it received the SGW change indication in the Context Acknowledge message in step 207.
Step 220
The SGW 110 acknowledges the message in step 219 by sending a Delete Session Response (Cause) messages to the old MME/old S4 SGSN 105a, 120a. The SGW 110 discards any packets buffered for the UE 101.
Step 221
The new MME 105b sends a TAU Accept message to the UE 101.
Step 222
If GUTI was included in the TAU Accept message in step 221, the UE 101 acknowledges the received message by returning a TAU Complete message to the new MME 105b. 
As seen in FIG. 2, the method used today for handling the TAU procedure is signaling intense. Signals are sent over the S10, S11 and S5/S8 as well as S6 interfaces. S10 is the interface between MMEs, S11 is the interface between the MME 105 and the SGW 110, S5/S8 is the interface between the SGW 110 and the PGW 118 and S6 is the interface between the MME 105 and the HSS 108.